Hydraulic power-assisted steering system

ABSTRACT

The invention relates to a hydraulic power-assisted steering system comprising a steering gear and a hydraulic actuator for assisting actuation of the steering wheel by the driver of a vehicle, in particular a motor vehicle, comprising an electric motor which is coupled to the steering column and used as an additional torque actuator for actively applying an additional steering torque, also consisting of an electronic control and regulating unit (ECU), which includes a determination unit for determining a steering torque and an evaluating and selecting circuit, by means of which a total value for applying the additional steering torque is determined in consideration of the determined steering torque or a quantity derived therefrom and a selected basic characteristic curve of steering (basic characteristic curve), with the total value of the additional steering torque to be applied including a driver-dependent component and a driver-independent component.

TECHNICAL FIELD

The present invention relates to a hydraulic power-assisted steeringsystem comprising a steering gear and a hydraulic actuator for assistingactuation of the steering wheel by the driver of a vehicle, inparticular a motor vehicle, comprising an electric motor which iscoupled to the steering column and used as an additional torque actuatorfor actively applying an additional steering torque, also consisting ofan electronic control and regulating unit (ECU).

The invention also relates to a method for controlling a hydraulicpower-assisted steering system, wherein the actuation of the steeringwheel by the driver of a vehicle, in particular a motor vehicle, isassisted by a hydraulic force or pressure, and wherein an electric motorwhich is coupled to the steering column is used as an additional torqueactuator for actively applying an additional steering torque by way ofan electronic control or regulation of the electric motor.

BACKGROUND OF THE INVENTION

Up-to-date motor vehicles, in particular passenger vehicles, aregenerally equipped with hydraulic or electrohydraulic power-assistedsteering systems, hereinbelow referred to as ‘hydraulic servo steeringsystems’. The servo assistance is devised such that actuators, e.g.hydraulic cylinders, are arranged in the mid-portion of the steeringmechanism. A force generated by the actuators assists in the actuationof the steering mechanism in response to the turning of the steeringwheel (torque assistance). This reduces the torque that must begenerated for steering the vehicle, and the driver is relieved in thesteering activity.

It is known in the art to devise the torque assistance or the boostingof the steering system in response to speed. At low vehicle speeds asthey are typical of pulling in or out of a parking space, easy-runningsteering with a high torque assistance is preferred, while heavysteering with a lower torque assistance is desirable when driving fast.

BRIEF SUMMARY OF THE INVENTION

An object of the invention is to devise a hydraulic power-assistedsteering system, which allows the provision of variable torqueassistance and is easy to integrate into already existing hydraulicpower-assisted steering systems.

This object is achieved by means of the features of the independentclaims. Dependent claims are directed to preferred embodiments of theinvention.

The term ‘steering wheel’ herein means all possible actuating devicesfor steering a vehicle such as a steering wheel or a joystick.

The essence of the invention is that the electronic control andregulating unit (ECU) includes a determination unit for determining asteering torque and an evaluating and selecting circuit, by means ofwhich a total value for applying the additional steering torque isdetermined in consideration of the determined steering torque or aquantity derived therefrom and a selected basic characteristic curve ofsteering (basic characteristic curve), and the total value of theadditional steering torque to be applied includes a driver-dependentcomponent and a driver-independent component.

Consequently, the additional steering torque includes a principallydriver-dependent component that corresponds to a basic steeringfunction, in particular a driving-speed-responsive steering assistance,e.g. steering defined by parameters. A principally driver-independentcomponent is provided as another component and used to realize driverassist functions. The latter means e.g. systems for the trackingstability of the vehicle (lane keeping) or for assistance in highlydynamic roadway situations, e.g. according to a driving dynamics system(ESP system).

The invention arranges that different predetermined characteristiccurves can be selected for varying the application of the additionalsteering torque.

According to the invention, the characteristic curve represents acharacteristic curve of amplification conveying the additional steeringtorque to be applied in dependence on the steering torque applied by thedriver and on an amplification factor.

It is provided by the invention that different predeterminedcharacteristic curves for the variation of the application of theadditional steering torque can be selected by way of a control variableST, which is directly or indirectly predefinable by the driver.

According to the invention, the electronic control and regulating unit(ECU) is designed redundantly.

According to the invention, the electric motor is coupled to thesteering column by way of a gear, preferably a belt drive.

According to the invention, the amplification factors of the differentcharacteristic curves are variable in response to the vehicle speed.

According to the invention, a steering recommendation is given to thedriver by means of the driver-independent component.

It is provided according to the invention that the vehicle is stabilizedand the vehicle dynamics is enhanced, respectively, by means of thedriver-independent component.

This embodiment is especially preferred because comfort and safety inhighly dynamic or safety-critical driving situations can be enhancedsignificantly by considering quantities related to driving dynamicsaccording to the invention.

It is arranged according to the invention that the additional steeringtorque is adapted to a vehicle course and a roadway course by means ofthe driver-independent component.

It is provided by the invention that the variations of the additionalsteering torque are achieved by using a scaling factor λ or anamplification factor V (where V=1/λ).

Preferably, the scaling factor λ or amplification factor V (where V=1/λ)is limited to a predefined value in a torque-dependent fashion. Thismeans that a limitation is introduced, in particular according to theratio between a steering torque that is maximally applicable by thedriver and a steering torque to be actually applied by the driver inorder that boosting of the steering activity in total will not fallbelow a defined value.

It is provided by the invention that the means for the activeapplication of the additional steering torque are designed as a unitthat is arranged as a module at a steering-wheel-side end portion of thesteering valve of a hydraulic power-assisted steering system.

The object is also achieved by a method characterized in that a totalvalue of the additional steering torque is determined from adriver-dependent component and a driver-independent component inconsideration of the additional steering torque or a quantity derivedtherefrom and a preselected basic characteristic curve of steering(characteristic curve).

Preferably, a variation of the application of the additional steeringtorque is executed in said method by way of selecting a characteristiccurve from several different predetermined characteristic curves.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an embodiment of the power-assistedsteering system according to the invention comprising anelectromechanical actuator.

FIG. 2 is a basic characteristic curve of a power-assisted steeringsystem and a modified characteristic curve.

FIG. 3 depicts a basic characteristic curve of a power-assisted steeringsystem and a modified characteristic curve with a center displacement.

FIG. 4 a illustrates a basic characteristic curve of a power-assistedsteering system and two characteristic curves modified in accordancewith the vehicle speed by means of an amplification factor V.

FIG. 4 b depicts a dependency of the amplification factor V inaccordance with the vehicle speed.

FIG. 5 illustrates an embodiment for determining a superposition torquefor actuating the electric motor of the power-assisted steering systemaccording to the invention.

FIG. 6 shows another embodiment for determining a superposition torquefor actuating the electric motor of the power-assisted steering systemaccording to the invention.

FIG. 7 is a schematic view of a second embodiment of the power-assistedsteering system of the invention comprising an electromechanicalactuator and the determination of the torque at the torsion rod on thebasis of signals of pressure sensors in the working chambers of thepower-assisted steering system.

FIG. 8 illustrates the determination of the torque at the torsion rod onthe basis of a determined hydraulic pressure in the working chambers ofthe power-assisted steering system.

FIG. 9 is a schematic view of a third embodiment of the power-assistedsteering system of the invention comprising an electromechanicalactuator and for the determination of the torque at the torsion rod onthe basis of signals of sensors for determining the angle of rotation atthe motor and at the steering wheel shaft of the power-assisted steeringsystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The steering system illustrated in FIG. 1 is composed of a steeringwheel 1, a steering column 2 with two universal joints 3, 4 connected tothe steering wheel 1. The steering column 2 is connected to or part of asteering wheel shaft 5, hereinbelow also referred to as ‘torsion rod’which actuates steering tie rods 8, 9 secured to the steering rack 7 byway of a steering gear 6, a steering rack 7, herein designed a steeringrack 7, and thereby causes turning of the wheels 10, 11. In thesteering-rack steering system shown herein, hydraulic assistance isrealized by means of a hydraulic pump 13 driven by the driving motor ofthe vehicle, e.g. by way of a belt drive 12, said pump supplyingpressurized fluid to a steering valve 14 through a conduit 15. Thepressure fluid can flow back into a supply reservoir 17 through a returnline 16. When the steering wheel adopts a straight position, a constantoil flow returns through the steering valve being in its neutralposition (open center) and through the return line 16. The pressure intwo chambers 18, 19 of a working cylinder 20 arranged at the steeringrack 7 will then have an equal amount. No steering assistance isexecuted. The steering rack 7 and, thus, also piston 21 is displacedwhen the steering wheel 1 is turned. The pressure of the pressure fluidassists the movement of piston 21. As this occurs, valve 14simultaneously causes pressure fluid to propagate from one chamber intothe other chamber so that hydraulic assistance is imparted to thesteering actuation in total.

For producing an additional torque, this conventional hydraulicpower-assisted steering system described hereinabove includes anelectric motor 23, a redundant control unit ECU 24 for actuating themotor 23 and for evaluating signals of a redundant steering torquesensor 25. Preferably, a sensor for the position of the motor 26 is alsoprovided. The electronic components are connected to an electric energysource 27. The control unit ECU 24 and the torque sensor preferably havea redundant design.

The steering wheel torque or steering torque, which is defined by thesteering system characteristics and the forces acting can be influencedactively by the E-motor 23 because said motor produces an additionaltorque (additional steering torque) and applies it to the steering rod.It is possible to add the torque to the constructively predeterminedsteering wheel torque or to deduct it therefrom. The motor torque can betransmitted with or without rear ratio, directly or by way of a gear 28,as illustrated herein. A belt drive, a helical gear/worm gear, or a spurgear system can be used to this end.

Upon failure of the E-motor 23 and/or the control unit ECU 24, thesteering characteristics with servo assistance known to the driver ismaintained. E-motor 23 and control unit ECU 24 are fail-silent for thispurpose. This means that these components are disconnected in a case ofmalfunction.

Advantageously, a conventional power-assisted steering system can beused without modifications to the characteristics by means of the systemfor producing an additional torque.

The invention renders it possible to vary the boosting characteristiccurve by producing an additional torque (see FIG. 2). The characteristiccurve of amplification herein means the dependency of the steering wheeltorque or actuating torque M on the existing system pressure P of thehydraulic assistance. The variations of the additional steering torqueare preferably effected by an amplification factor V or scaling factorλ, and V=1/λ applies. The torque A (M_steering) is reduced by activelyapplying an additional torque B (M_motor) in the embodiment shown. Thismeans that the driver is more significantly assisted in his/her steeringactivity. The result is a modified characteristic curve (dotted curveK_(B)) with a lower actuating force compared to the originalcharacteristic curve (solid curve K_(A)) . Active driver assistance canbe realized by this variation of the characteristic curve. A steeringrecommendation is given by the torque variation.

In addition, active driver assistance is realized in a particularlyfavorable manner by way of a displacement of the central point of acharacteristic curve (center displacement), e.g. the characteristiccurve K_(A) or K_(B). FIG. 3 depicts how a center displacement isexecuted in addition to a torque variation shown in FIG. 2. Similarly toFIG. 2, the torque A (M_steering) is reduced by the active applicationof the additional torque C (M_motor). The resulting characteristic curveK_(C) (dotted curve in FIG. 3) with a lower actuating force compared tothe original characteristic curve (solid curve K_(A)) herein correspondsto the modified characteristic curve K_(B) as shown in FIG. 2 (dottedcurve in FIG. 3 and FIG. 2) with an additional displacement of thecenter M_CF,0. This imparts a stronger assistance to the steeringactivity of the driver in a certain direction giving an indication inwhich direction the driver shall steer. The steering recommendation tothe driver, as to in which direction he/she shall steer, can be givenespecially favorably with this system.

It is shown in FIG. 4 a how a basic characteristic curve K_(A) is variedin accordance with the vehicle speed V_(veh). In the embodiment shown,the basic characteristic curve K_(A) is modified by an amplificationfactor V (1/λ) greater than 1, herein e.g. a factor 2, at a lowervehicle speed V_(veh) when the vehicle speed is below a first limitvalue V_(veh,1), i.e. when V_(veh)<V_(veh,1) applies. The result is amodified speed-corrected characteristic curve (characteristic curveK_(D) in FIG. 4 a) . On the other hand, the basic characteristic curveK_(A) is modified by an amplification factor V (1/λ) smaller than 1,herein e.g. a factor 1/2, at a higher vehicle speed V_(veh), when thevehicle speed is above a second limit value V_(veh,n), i.e whenV_(veh)>V_(veh,n). The result is a modified, speed-correctedcharacteristic curve (characteristic curve K_(E) in FIG. 4 a).

The variation of the amplification factor V (1/λ) in accordance withspeed is shown in FIG. 4 b. Up to the first limit value of the vehiclespeed V_(veh,1) the amplification factor V remains at a first, highervalue V₁, herein e.g. a factor 2. It will then decrease continuously atrising vehicle speed V_(veh) (control range R) and reach a second, lowervalue V_(n), herein e.g. a factor 1/2, starting from the second limitvalue of the vehicle speed V_(veh,n), e.g. when V_(veh)>V_(veh,n)applies.

In the embodiment of the actuation of the electric motor of thepower-assisted steering system of the invention as shown in FIG. 5, thesteering wheel torque of the driver is sensed by means of a torquesensor.

A variation of the steering assistance can take place e.g. in dependenceon the vehicle speed (similar to steering defined by parameters), as isshown in FIGS. 4 a and 4 b. The variation is preferably executed by wayof the amplification factor V according to the invention. However, othercalculation rules are also feasible.

Input quantities for the determination are a torque M_(driver) 31 to beapplied by the driver, a torque M_(Dr,max) 32 maximally applicable bythe driver, the vehicle speed V_(veh) 33, and a control variable ST 34predefinable directly or indirectly by the driver. Differentcharacteristic curves and possible variations can favorably be selectedby means of the control variable ST 34.

In accordance with the vehicle speed V_(veh) 33 and the control variableST 34, an amplification factor V 37 is determined in a determinationunit 35, preferably according to a predetermined or predeterminablefunction 36.

A limiter function 38 is used in the amplification factor V,corresponding to 1/λ, to take into account also a minimum value for theamplification factor V 39, which results 40 from the ratio between thetorque M_(driver) 31 to be applied by the driver and the torqueM_(Dr,max) 32 that is maximally applicable by the driver. Thistorque-dependent limitation of the amplification factor V prevents thatthe torque M_(Dr,max) 32 to be applied by the driver rises excessivelyand, as the case may be, uncontrolledly when the steering assistance isreduced, at an amplification factor V lower than ‘one’ (V<1).

For standardization purposes, the value ‘one’ is deducted (V−1) 42 fromthe so determined amplification factor V 41, and the modifiedamplification factor (V_(mod)−1) 43 reduced by ‘one’ is superimposed 44on the torque M_(driver) 31 to be applied by the driver. With respect tothis superimposed torque 45, a variable torque M_(CF,0) 46 isadditionally taken into account 47, as the case may be, especially forthe purpose of center displacement. Advantageously, torque interventionsindependent of the driver are favorably taken into account by thevariable torque M_(CF,0), with the result of a displacement of the basiccharacteristic curve of steering. Active driver assistance on a torquebasis can be realized this way. Said driver assistance acts in the senseof a steering recommendation for the driver by way of a higher-rankingcontrol system (not shown). On the other hand, steering in one directioncan be impaired by a torque increase. In the case of a steeringrecommendation for the driver, the additional torque M_(CF,0) 46 isincreased by a defined delta torque in the direction of the targetcourse until the steering angle (corresponds to the target course of thevehicle) calculated by the higher ranking control system has beenadjusted by the driver (center displacement). When the driver ignoresthe steering recommendation by not following the desired specifications,the additional torques M_(CF,0) 46 calculated on the basis of thesteering recommendation will be reduced slowly again, that means,gradually to the value ‘zero’ (M_(CF,0) =0) . An appropriatespecification of the additional torque renders it possible to warn ofcritical driving situations.

A resulting motor torque M_(Mot) 48 is produced as an output quantity.

FIG. 6 depicts another embodiment of the invention wherein the use of atorque sensor for sensing the hand torque of the driver M_(driver) isomitted. The torque of the torsion rod of the steering system that isnecessary for the torque superposition function is reconstructed. Adetermined or estimated hand torque of the driver M_(Dr,mean) 51 is usedinstead of the torque M_(driver) 31 to be applied by the driver as aninput quantity in FIG. 5, in addition to the torque M_(Dr,max) 52 thatis maximally applicable by the driver, the vehicle speed V_(veh) 53, thecontrol variable ST 54 that is directly or indirectly predefinable bythe driver.

A scaling factor λ 57, corresponding to an amplification factor V(V=1/λ) is determined in the determination unit 55 on the basis of thevehicle speed V_(veh) 53 and the control variable ST 54 according to apreferably predetermined or predeterminable function 56.

A maximum scaling factor λ_(max) is determined 60 from the establishedor estimated hand torque of the driver M_(Dr,mean) 51 and the torqueM_(Dr,max) 52 maximally applicable by the driver. Said value is used asinput quantity 59 for the limiter function 58. This limitation of thescaling factor λ by said function 58 prevents that the torque M_(Dr,max)52 rises excessively and, as the case may be, uncontrolledly, in theevent of a scaling factor λ higher than ‘one’ (λ>1) and, hence, anamplification factor V lower than ‘zero’ (V<0).

For standardization purposes, the so determined scaling factor λ 61 isdeducted from the value ‘one’ (1−λ_(mod)) 62 and the so standardizedvalue (1−λ_(mod)) 63 is superimposed 64 on the determined or estimatedvalue M_(Dr,mean) 51 for the torque applicable by the driver.

The variable torque M_(CF,0) 66 independent of the driver issuperimposed 67 in addition, as the case may be, to the superimposedtorque 65 , in particular for the purpose of center displacement. Theresult is the output value for the superposition torque M_(Mot) 68.

FIG. 7 illustrates a second embodiment of the power-assisted steeringsystem of the invention comprising an electromechanical actuator,wherein a torque at the torsion rod 5 is determined on the basis ofsignals of pressure sensors 71, 72 in the hydraulic working chambers 18,19 of the power-assisted steering system. The basic steering system withhydraulic assistance is identical with the system shown in FIG. 1 and,therefore, will not be explained in detail.

FIG. 8 displays the determination of the torque at the torsion rod onthe basis of a determined hydraulic pressure in the working chambers ofthe power-assisted steering system. The (hydraulic) servo pressureP_(servo) results from the difference between the hydraulic pressure inthe right working chamber P_(WC,right) and in the left working chamberP_(WC,left) (P_(servo)=P_(WC,right)−P_(WC,left)) . The correspondingcharacteristic curve of the basic steering function is plotted as solidcurve K_(A) in FIG. 8. An associated torque M_(Dr,mean) at the torsionrod can be determined from a defined value for the servo pressureP_(Servo), herein e.g. P_(M) in FIG. 8, by way of the characteristiccurve K_(A). The dotted curve represents the above-described (see FIG.2) characteristic curve with a positive servo pressure for a rotation tothe right (right-hand half) and a rotation to the left (left-hand half).It therefore corresponds to the amount of the measured difference inpressure P_(WC,right)−P_(WC,left).

FIG. 9 represents a third embodiment of the power-assisted steeringsystem of the invention comprising an electromechanical actuator and thedetermination of the torque at the torsion rod on the basis of signalsfrom the sensors determining the angle of rotation at the motor 26(motor angle φ_(Mot)) and another sensor determining the angle ofrotation 73 at the steering wheel shaft (pinion angle δ_(T)) of thepower-assisted steering system. The basic steering system with hydraulicassistance is identical with the system shown in FIG. 1 and, therefore,will not be explained in detail. The torque at the torsion rod 5 isdetermined by measuring the difference of angles on the basis of themeasuring signals of the sensors 26, 73.

To this end, the steering wheel angle δ_(H) is derived from the motorangle φ_(Mot) according to the relation δ_(H)=ν*φ_(Mot). The torque atthe torsion rod M_(Dr) or M_(torsion rod) is then determined by means ofthe known rigidity of the torsion rod 5 C_(T) and the measureddifference of angles according to the relationM_(Dr,mean)=C_(T)*(δ_(H)−δ_(T))

1-15. (canceled)
 16. Hydraulic power-assisted steering system comprisinga steering gear and a hydraulic actuator for assisting actuation of thesteering wheel by the driver of a vehicle, comprising an electric motorwhich is coupled to the steering column and used as an additional torqueactuator for actively applying an additional steering torque, alsoconsisting of an electronic control and regulating unit (ECU), whereinthe electronic control and regulating unit (ECU) includes adetermination unit for determining a steering torque and an evaluatingand selecting circuit, by means of which a total value for applying theadditional steering torque is determined in consideration of thedetermined steering torque or a quantity derived therefrom and aselected basic characteristic curve of steering (basic characteristiccurve), and the total value of the additional steering torque to beapplied includes a driver-dependent component and a driver-independentcomponent.
 17. Power-assisted steering system as claimed in claim 16,wherein different predetermined characteristic curves can be selectedfor varying the application of the additional steering torque. 18.Power-assisted steering system as claimed in claim 16, wherein thecharacteristic curve represents a characteristic curve of amplificationconveying the additional steering torque to be applied in dependence onthe steering torque applied by the driver and on an amplificationfactor.
 19. Power-assisted steering system as claimed in claim 16,wherein different predetermined characteristic curves for the variationof the application of the additional steering torque can be selected byway of a control variable ST that is directly or indirectly predefinableby the driver.
 20. Power-assisted steering system as claimed in claim16, wherein the electronic control and regulating unit (ECU) is designedredundantly.
 21. Power-assisted steering system as claimed in claim 16,wherein the electric motor is coupled to the steering column by way of agear.
 22. Power-assisted steering system as claimed in claim 16, whereinthe amplification factors of the different characteristic curves arevariable in response to the vehicle speed.
 23. Power-assisted steeringsystem as claimed in claim 16, wherein a steering recommendation isgiven to the driver by means of the driver-independent component. 24.Power-assisted steering system as claimed in claim 16, wherein thevehicle is stabilized and the vehicle dynamics is enhanced,respectively, by means of the driver-independent component. 25.Power-assisted steering system as claimed in claim 16, wherein theadditional steering torque is adapted to a vehicle course and a roadwaycourse by means of the driver-independent component.
 26. Power-assistedsteering system as claimed in claim 16, wherein the variations of theadditional steering torque are effected by using a scaling factor λ oran amplification factor V, respectively, according to the relationV=1/λ.
 27. Power-assisted steering system as claimed in claim 16, thepower-assisted steering system further comprising the variations of theadditional steering torque are effected by using a scaling factor λ oran amplification factor V, respectively, according to the relationV=1/λ, wherein the scaling factor λ or amplification factor V (whereV=1/λ) is limited to a predefined value in a torque-dependent fashion.28. Power-assisted steering system as claimed in claim 16, wherein themeans for the active application of the additional steering torque aredesigned as a unit that is arranged as a module at a steering-wheel-sideend portion of the steering valve of a hydraulic power-assisted steeringsystem.
 29. Method of controlling a hydraulic power-assisted steeringsystem in which the actuation of the steering wheel by the driver of avehicle is assisted by a hydraulic force or pressure and an electricmotor which is coupled to the steering column is used as an additionaltorque actuator for actively applying an additional steering torque byway of an electronic control or regulation of the electric motor,wherein a total value of the additional steering torque is determinedfrom a driver-dependent component and a driver-independent component inconsideration of the additional steering torque or a quantity derivedtherefrom and a preselected basic characteristic curve of steering(characteristic curve).
 30. Method as claimed in claim 29, wherein avariation of the application of the additional steering torque isexecuted by way of selecting a characteristic curve from severaldifferent predetermined characteristic curves.